Pyrroloquinoline quinone (hereinafter sometimes referred to as “PQQ”) has been proposed as a possible new vitamin (see, for example, Non-patent document 1), and has attracted much attention as a useful material for dietary supplements, cosmetics, etc. Moreover, PQQ is present not only in bacteria but also in eukaryotic molds and yeasts and plays an important role as a cofactor. Also, PQQ has been found to have many physiological activities such as cell growth-promoting activity, anti-cataract activity, hepatic disease-preventing and therapeutic activity, wound healing activity, antiallergic activity, reverse transcriptase-inhibiting activity and glyoxalase I-inhibiting activity—anticancer activity, nerve fiber regeneration, and the like.
PQQ can be obtained as an alkali metal salt form by subjecting a product obtained by methodologies such as organic chemical syntheses (Non-patent document 2) or fermentation processes (Patent document 1) to chromatography, concentrating the PQQ fraction in the effluent and adding an organic solvent to the fraction followed by crystallization (Patent document 2). This alkali metal salt of PQQ, which has been known as a water soluble substance, in fact has low solubility in water, and PQQ in the free form has lower solubility in water. Moreover, the solubility of the alkali metal salt of PQQ in alcohol is even lower than in water. The alkali metal salt of PQQ has a low solubility because of the presence of molecular interactions due to hydrogen bonds and ionic bonds (Non-patent document 3). This is why it is difficult to prepare an aqueous solution having a high concentration of an alkali metal salt of PQQ and even more difficult to provide a solution of the alkali metal salt of PQQ in an organic solvent.
Solutions of PQQ in water and in ethanol, however, are most commonly used when provided for use in the food and pharmaceutical fields, so that PQQ is required to be easily dissolved at a high concentration in such a solvent and to remain unprecipitated. Also in separation and purification of PQQ, its low solubility entails disadvantages of requiring large amounts of a solvent (water in particular) for dissolving it and thus large equipment, and producing large amounts of waste water. Moreover, PQQ is required to be soluble in a solvent having a higher vapor pressure because water needs much heat to evaporate, which results in higher costs of fuel. From these, there is a need for PQQ having an increased solubility.
Furthermore, PQQ has a quinone structure, which is readily reduced (Non-patent document 4), and this reduced PQQ has also attracted attention by its functions (Non-patent document 5).